In a technical field of radio communication, a space division multiplexing (SDM) scheme is often used in order to increase an information amount that is allowed to be transmitted or received at the same time. The SDM scheme is achieved by a multiple input multiple output (MIMO) scheme in which a plurality of antenna elements are used in each of a transmitter and a receiver (for example, see Japanese Laid-open Patent Publication No. 2012-253709).
In addition, there is a strong demand in the market for performing high-speed radio communication in a millimeter wave band having abundant available spectrums. The communication through the millimeter wave band is performed in a line-of-sight (LOS) environment because the signal wavelength is short. Thus, from a viewpoint of high-speed large-capacity communication or the like, it is conceivable that communication is performed by the MIMO scheme in the LOS environment using the millimeter wave band. The communication scheme may be used for various pieces of radio communication, and for example, it is desirable that such a communication scheme is used for communication between fixed communication stations such as communication between base stations in a mobile communication system, back-haul communication, and communication with a relay station. This is because the LOS-MIMO scheme allows the communication between communication stations to be performed at high speed even in a situation in which it is difficult to lay a cable between the communication stations. Such a situation includes, for example, a case in which communication is performed across a river and a case in which communication is performed at a certain location in the event of a disaster.
In the LOS-MIMO scheme of the related art, a plurality of antenna elements that are included in each of a transmitter and a receiver are disposed in a row (in a straight line) at equal intervals by a certain distance d. The distance d is represented by “d=√(λR/N)”, where “R” represents a range between the transmitter and the receiver (range between the antenna element in the transmitter and the antenna element in the receiver), “λ,” represents a wavelength of a signal to be transmitted or received, and “N” represents the number of antenna elements. In a case in which the plurality of antenna elements are arranged at equal intervals by the distance d in each of the transmitter and the receiver, when the range between the transmitter and the receiver is the range R or is within a certain extent that includes the range R, the transmitter and the receiver may perform communication with an appropriate quality. Such extent corresponds to a communication guaranteed range in which communication with an appropriate quality is guaranteed between the transmitter and the receiver.
For a transmitter and a receiver that communicate with each other across a range R′ outside of the communication guaranteed range, it is desirable that the design is started over. That is, it is desirable that an appropriate distance d′ between the antenna elements, which corresponds to the range R′, is calculated, and the antenna elements are arranged at equal intervals by the distance d′. The recalculation of the distance between the antenna elements, the rearrangement of the antenna elements, and the like, are time-consuming and troublesome. Thus, from a viewpoint to avoid such time and trouble, it is desirable that the communication guaranteed range is wide. When the communication guaranteed range is wide, the flexibility in arrangement of the transmitter and the receiver is high. However, in the case of the transmitter and the receiver in the LOS-MIMO scheme of the related art, the communication guaranteed range is not sufficiently wide, and locations for arranging the transmitter and the receiver are quite limited undesirably.